Adhesive is used extensively in many industries as an efficient means of joining two or more elements. In the electronics industry, for example, adhesives are particularly used for supporting components on printed circuit boards. The adhesives may additionally exhibit heat-conducting or electrical-insulation properties when the adhesive has dispersed therein small particles, or filler, which impart the desired characteristic. For example, heat conducting adhesives, which help conduct heat away from the components on a circuit board, include metallic particles dispersed therein to aid in heat transfer through the adhesive. Dielectric adhesives, which help to electrically insulate the adhered components, include small glass spheres, on the order of about 1/5000 of an inch in diameter, dispersed therein to provide sufficient spacing in the adhesive so as to disrupt electrical conduction therethrough.
Automated dispense systems ordinarily use high pressure to deliver filled adhesives through a controllable dispense valve. The dispense valve can be opened to provide the filled adhesive to the objective surface and closed when sufficient adhesive has been applied. The predominate difficulty when dispensing filled adhesives in this manner is posed by blockage of the dispensing system by the adhesive fluid. Blockages result from either the premature curing of the adhesive or the pack-out of the filler material within the delivery system.
Premature curing of the adhesive refers to the hardening or drying of the adhesive while still within the delivery system. As curable adhesives may be sensitive to time, temperature, and humidity, the adhesive may cure in the delivery system without being dispensed. Premature curing can block the flow of un-cured adhesive through the delivery system requiring a system shutdown until the cured adhesive is cleared out.
Pack-out of the filler material refers to the tendency of the filler particles to separate from the adhesive. Pack-out usually occurs when a filled adhesive is subjected to shearing stresses resulting from the filled adhesive being forced through geometrically varying delivery components, particularly at the areas where the delivery components geometrically constrict, such as at nozzles, pressure regulators, orifices, valves, hard comers, and the like. Pack-out at a pressure regulator during the static state, i.e. when the dispense valve remains closed for long periods, for example, results in pressure creep downstream of the regulator as the regulator is hampered in reducing fluid pressure in the lines. Pack-out during the dynamic state, i.e. when the dispense valve is periodically opened and closed, may occur at points of restriction and result in a diminished or inconsistent flow rate through the system. Furthermore, pack-out occurring at the dispense valve itself may cause the dispense valve to fail in an open condition and result in an uncommanded fluid dispensement.
As a result of blockages from premature curing and pack-out, the dispensing system must be shut down and cleaned of the restricting blockages. Shutting down the delivery system for such maintenance can unacceptably delay full production of the target article. Furthermore regularly scheduled maintenance alone is an insufficient safeguard as both premature curing and pack-out are neither periodic nor predictable. There is therefore a need for a curable fluid or filled-fluid delivery system that can self-diagnose indicia of premature curing and pack-out and can allow an operator to clear any blockages while minimizing the down-time resulting therefrom.